Ion source



1956 l. H. MCLAREN ET AL ION SOURCE 2 Sheets-Sheet 1 Filed April 18.1951 R E E Him 0\ m m 9 6 3 a a E 3 INVENTOR. /4/V ,7. MCLAREA/ BYW/LL/AM c. W/AEY M & MB Arron/5y Jan. 24, 1956 H. M LAREN ET AL2,732,500

ION SOURCE Filed April 18, 1951 2 Sheets-Sheet 2 DELA Y L/NE PUL 5EfORM/NO- C/RCU/T INVEN TOR.

[M M M: ZAKEA/ BY W/LL/A/l I C. W/LEY 2:; g. .5 ATTORNEY United StatesPatent ION SOURCE Ian H. McLaren, Dearborn, and William C. Wiley, De-

troit, Mich., assignors to Bendix Aviation Corporation, Detroit, Mich.,a corporation of Delaware Application April 18, 1951, Serial No. 221,554

21 Claims. '(Cl. 250-41.9)

This invention relates to apparatus for, and methods of, producing abeam of ions and more particularly to apparatus for, and methods of,producing a beam of ions in pulse form. The invention is especiallyadapted for use in conjunction with a mass spectrometer to producepulses of ions from the gases in an unknown mixture.

A mass spectrometer is known which has a magnetic field of constantstrength to rotate ions of different mass through circular paths of oneor more complete revolutions. As they rotate, the ions of relativelysmall mass travel faster than the ions of larger mass. Thus, the ionsarrive at a collector assembly at times directly dependent upon theirmass. By measuring the times of flight of the different ions, the massof the ions can be easily determined.

Since the masses of different ions are measured on the basis of theirtime of flight, a continuous supply of ions to the mass spectrometerwould prevent a physical separation between the ions of different mass.Because of this, pulses of ions must be produced in order for thespectrometer to operate properly. The ion sources now being used toproduce pulses of ions for introduction to the mass spectrometer do notoperate entirely satisfactorily. For example, the pulses produced by theion sources now in use are not sharply delineated and are relativelyweak. As a result, they hinder clear-cut and precise measurements ofmass'from being made by the spectrometer.

This invention provides an ion source which produces pulses of ionshaving strengths far in excess of the pulse strengths produced by thesources now in use. In addition to producing pulses having relativelylarge numbers of ions, the source also concentrates the ions in eachpulse into a clearly defined space so that each pulse of ions is sharplydelineated with respect to all other pulses. Concentrating the ions ineach pulse into a small space also increases the accuracy in theoperation of the spectrometer into which the ions may be introduced.

An object of this invention is to provide an ion source for producingsharply delineated pulses of ions.

Another object of the invention is to provide an ion source of theabove-character for producing ion pulses having a maximum strength.

A further object is to provide an ion source of the above character forconcentrating a maximum number of ions into a minimum space to produce asharp and strong pulse. I

Still another object is to provide a method'of producing strong andsharply defined pulses of ions.

Other objects and advantages will be apparent from a detaileddescription of the invention and from the appended drawings and claims.

In the drawings:

Figure l is a side elevational view, as seen from the right side, of anion source constituting oneembodiment of the invention and of a samplevchamber for introducing gases to the ion source for the production ofions;

Figures 2 and 3 are sectional views substantially on 'ice the lines 22and 3-3, respectively, of Figure l, illustrating the ion source infurther detail;

Figure 4 is a sectional view substantially on the line 44 of Figure 3;

Figure 5 is a perspective view illustrating the grid for withdrawing theions in pulses from the ion source;

Figure 6 is a simplified diagram of the electrical circuit used to applythe proper voltages to the various grids in the ion source shown in theprevious figures;

Figure 7 is a perspective view illustrating a mass spectrometer withwhich the ion source shown in the previous figures may be used andfurther illustrating the relative position of the ion source in thespectrometer;

Figure 8 is a perspective view showing particular features of thespectrometer illustrated in Figure 7 in further detail; and

Figure 9 is a schematic diagram illustrating a representative travelpath followed in the spectrometer by the ions which are produced by theion sources.

A mass spectrometer, generally illustrated at 10 in Figures 7 and 8, isadapted to receive pulses of ions from an ion source, generallyindicated at 12 and illustrated in detail in Figures 1 to 5, inclusive.The mass spectrometer 10 is in general cylindrically shaped and isdisposed between opposite poles of a magnet 14 (Figure 8) which isadapted to provide a magnetic field of uniform strength axially alongthe length of the cylinder. The ion source 12 and a collector assembly16 (Figures 7 and 9) are both disposed within the spectrometer inrelatively close angular relationship to each other and in closelyaligned axial relationship to each other, for reasons which will beexplained in detail hereinafter.

The source 12 operates upon the difierent gases constituting an unknownmixture to produce ions from the gases. Each ion has a mass which isproportionate to the mass of the gas from which it is produced. The ionsare given an initial velocity by the source 12 and are then emitted fromthe source in a direction perpendicular to the magnetic field created bythe magnet 14. This causes the magnetic field to exert a force upon theions in a direction perpendicular to the direction of ion travel and theions todescribe a circular path, as illustrated at 18 in Figure 9.

Since the emitting force exerted by the ion source 12 is substantiallyconstant, this force has a greater eifeet upon the ions of relativelylight mass than upon the ions of relatively heavy mass. Thus, each ionis given an initial acceleration which is substantially inverselyproportional to its mass.

r. M where a=the acceleration at which an ion leaves the source 10; M=the mass of the ion; and K =a constant.

Because of their greater initial acceleration, the ions of relativelylow mass rotate through a complete revolution faster than the ions ofrelatively heavy mass and reach the collector plate 16 before the ionsof heavy mass. By measuring the time required for the ions to describeone or more substantially complete revolutions, the masses of thedifferent ions can be determined. The abundance ratio of the differentconstituents in the unknown mixture can also be determined by measuringthe relative amplitudes of the signals produced at the collector plate16 by the ions formed from each constituent in the mixture.

In order to measure the times of flight required for ions of differentmass, pulses of ions must be provided by the ion source 12, whichincludes a base (Figure 2) having a relatively thin portion slotted asat 22. A block 24 suitably secured to a wall as at 26 is positionedbelow the thin portion of the base 20 and a pedestal 28 is supported ontop of the base. The block 24 and pedestal 28 have vertical bores whichare aligned with the slot 22, and a bolt 30 extends through the boresand the slot to secure the pedestal and block to the base. Because ofthe slot 22, the base 20 may be moved relative to the block 24 and thepedestal For example, the base 20 may be moved to the left or right inFigure 2 and also, the base may be pivoted about the bolt 30 which isfixedly held by the block 24. Horizontally aligned bores are alsoprovided in the block 24 and in the thickportion of the base 20 so thata bolt 32 may extend through the bores. The bolt 32 supports a spring 34in compression between the block 24 and the base 20. A second bolt 36(Figures 1 and 2) extends through a threaded bore in the thick portionof the base 20 and presses against a wall 38. A compressed spring 40(Figures 2 and 4) helps to press the bolt 36 against the wall 38 sincethe base 20 pivots about the bolt 38.

An insulating plate 42 (Figures 1 and 2) slotted as at 44 is secured tothe base 20 as by screws 46. Screws 48 extend into the plate 42 andpress against the plate a clamp 50 holding filament leads 52. The leads52 extend upwardly into a housing 54 suitably closed at one end by aplate 55. The leads are connected within the housing 54 to awedge-shaped strip 56 of tungsten, which serves as a filament for theemission of electrons.

A control grid 58 (Figures 2 and 3), made from a sheet of suitableconductive material such as brass and having a centrally disposed hole60, is secured as by glass studs 62 to the housing 54 at the oppositeend of the chassis from the plate 55. The studs 62 also support anelectron accelerator grid 64 which is also made from brass and which isspaced from the control grid 58 as by mica washers 66. A hole 68 iscentrally provided in the electron accelerator grid 64 in alignment withthe hole in the control grid 58.

The control grid 58 and acceleration grid 64 are disposed above thepedestal 28. Flanges 70 (Figures 3 and 4) on a substantially U-shapedcasing 72 are supported as by studs 74 in the pedestal 28 and areprovided with elongated holes 76 for slidable positioning relative tothe pedestal 28. A slot 78 is provided in one side of the cover 72 andan ion accelerator grid 80 (Figures 4 and 5 is provided in juxtapositionto the slot 78. The grid 80 includes a plurality of equally spacedcopper wires vertically supported between two rods 81, the rods in turnbeing pinned against the casing 72 by brackets 82 and being suitablyinsulated from the casing.

A semi-cylindrical groove-84 (Figures 2, 3 and 4) is provided in the topof the pedestal 28 in longitudinal alignment with the holes 60 and 68 inthe grids 58 and 64, respectively. The groove 84 communicates with ahole 86 (Figure 4) which extends through the pedestal 28 in a transversedirection relative to the groove and which in turn communicates with apipe 88 push fit into the base. The pipe 88 and the hole 86 provide acontinuous circuit for the flow of fluid between the groove 84 and asample chamber 90 (Figure 1) containing the diflerent gases in anunknown mixture.

Studs 92 (Figure 3) also fit into the pedestal 28 and support flangesextending from a casing 96. One side of the casing 96 has a hole 98aligned with the holes 60 and 68 in the grids 58 and 64, respectively.The opposite side of the casing has a collector plate 108 secured to itin insulated relationship.

The circuit shown in Figure '6 applies the proper vole ages to thefilament 56, the grids 58, 64 and 80 and the collector plate 100. Thecircuit includesa resistance 102 and a potentiometer 104 in series witha suitable power supply, such as a battery 106, the positive terminal ofwhich is grounded. The filament 56 is connected to the common terminalbetween the resistance I02 and potentiometer 104, the control grid 58 tothe movable contact of the potentiometer 104, and the accelerator grid64 to the grounded positive terminal of the battery 106. In practice,approximately 90 volts may be applied to the filament 56 andapproximately 70 volts to the control grid 58. The collector plate 180is connected to the positive terminal of a suitable power supply, suchas a battery 108, supplying approximately +30 volts. The negativeterminal of the battery 108 is grounded.

In addition to being connected to the movable con tact of thepotentiometer 104, the control grid 58 is also connected through aresistance 110 to the output terminal of a pulse forming circuit 112 soas to receive at predetermined times a pulse of approximately l00 voltsand approximately I microsecond or less in length. The output pulse fromthe circuit 112 is also introduced through a suitable delay line 114 tothe ion accelerator grid 80, which receives a pulse of approximately-200 volts, preferably at approximately the same time as theintroduction of a pulse to the control grid 58 as will be disclosed indetail hereinafter.

Before the emission of any ions from the source 12, the filament 56 isadjusted in position relative to the control grid 58 and the electronaccelerator grid 64. The distance of the filament from the grids 58 and64 is controlled by the bolt 32. For example, when the bolt 32 is turnedin a clockwise direction, it moves the base 20 to the left in Figure 2and brings the filament 56 closer to the control grid 58. In likemanner, the lateral position of the filament 56 relative to the holes 60and 68 in the grids 58 and 64, respectively, can be varied by rotatingthe bolt 36. For example, the filament moves towards the right in Figure1 when the bolt 36 is turned in a clockwise direction, since the thickportion of the base 20 will move away from the wall 38 and theinsulating plate 42 will move with the base and carry the filament 56with it. The actual movement of the filament 56 will be along an arcsince the base 20 pivots on the bolt 30. Because of the slightadjustments required in the lateral positioning of the filament 56, itsmovements along the are are small and are substantially straight-linemovements. The vertical position of the filament relative to the holes60 and 68 can be adjusted by loosening the screws 46 and moving theinsulating plate 42 upwardly or downwardly before the screws aretightened.

After-the filament 56 has been properly positioned relative to the grids58 and 64, it is heated by current flowing through it and the leads 52to a temperature which produces an emission of electrons. The electronsare attracted towards the control grid 58 by the voltage on the grid andare concentrated into a narrow beam by the hole 60 in the grid. Afterpassing through the hole 60 in the grid 58, the electrons areaccelerated towards the collector plate by the voltages on theaccelerator grid 64 and on the collector plate and are furtherconcentrated into a narrow beam by the hole 68 in the accelerator gridand the hole 98 in the casing 96. Since the voltages on the control grid58, the accelerator grid 64 and the collector plate 100 becomeprogressively positive, the electrons increase in speed as they travelfrom the filament to the collector plate. A magnetic field may beapplied in the same direction as the electron fiow to assist in formingand maintaining a thin stream of electrons.

As the electrons travel through the space above the groove 84, theycollide with gas molecules flowing from the sample chamber 90 throughthe pipe 88 and upwardly into the groove. Because of their relativelyhigh speed, the electrons strike some of the gas molecules withsufiicie'n't force to produce ionization of these molecules. Since mostof these ions have a positive charge and the electrons have a negativecharge, the ions are retained in the potential well created by theelectron stream. This potential well initially has a large negativepotential because of the relatively large number of electrons flowingtowards the collector plate 100 and thus is able to retain a relativelylarge number of ions before its potential is neutralized.

As the number of ions in the potential well approaches saturation, theelectron stream is cut off, preferably by applying a negative pulse ofapproximately 100 volts on the control grid 58 from the pulse formingcircuit 112 (Figure 6). Since only the ions remain in the space abovethe groove 84 after the cut-ofi of the electron stream, a strongelectric field of positive potential is created. Upon the application ofa negative pulse of approxi mately 200 volts on the ion acceleratorgrid, a potential dilference is produced between the ion field and thegrid 80 to provide a strong attraction of the ions towards the grid.This causes the ions to travel in a pulse or bunch towards the ionaccelerator grid and beyond the grid into the magnetic field of the massspectrometer shown in Figures 7 and 8. As previously disclosed, the ionsof different mass in each pulse become separated as they describe acircular path in the spectrometer and reach the collector assembly 16(Figure 7) at different times, the times of flight providing anindication of the mass of the ions.

The negative pulse of approximately 200 volts is preferably applied onthe ion accelerator grid 80 at approximately the same time as theelectron stream from the filament 56 is cut off. It may also be appliedshortly after the cut-off of the electron stream, and it may sometimeseven be applied shortly before the introduction of a pulse to thecontrol grid 58. As previously disclosed, a negative pulse is preferablyapplied to the control grid 58 to return the electrons to the filament56, but a positive pulse may also be applied to the grid 58 to collectthe electrons at the grid. The filament 56 may also be normallymaintained at approximately ground potential to prevent the flow of anyelectrons and may be pulsed to a negative potential at such times aspulses of electrons are desired. The polarity'of the pulse applied tothe control grid 58 and the relative timing between the pulses on thecontrol grid and on the ion accelerator grid 80 are somewhat dependenton the use which is made of the pulses of ions. In other words, thepulse polarities and time relationships are dependent somewhat uponwhether the ion pulses are used in a mass spectrometer or in some otherapplication.

The ion source disclosed above has several important advantages. Byretaining the ions which are produced within the potential well createdby the electron stream, the number of ions held within a relativelysmall space is greatly increased over the capacity of equipment now inuse. Suddenly cutting ofi the pulse stream makes this vast reservoir ofions available in pulse form to apparatus such as a mass spectrometer.For example, we have found that the number of ions retained within thepotential well created by the electrons is approximately 1,000 to1,000,000 times greater than the number of ions which would normally bein the same space if norpotential well existed. This increase in thenumber of ions facilitates the measurement of the ion masses in aspectrometer and materially alleviates any difiiculty in the design ofamplifiers and other equipment associatedwith the spectrometer.

The potential well created by the electron stream may be made as narrowas desired by varying the cross-sectional areas of the holes 60 and 68in the control grid 58 and the ion accelerator grid 64, respectively.Producing a narrow potential well is desirable since it in turn causesan ion pulse of relatively short length to be produced. When introducedto a mass spectrometer, an ion pulse of short length facilitates a sharpditierentiation betweenthe pulsesof different mass. 7

As previously disclosed, the control grid 58 and ion accelerator grid 80are usually pulsed when the number of ions in the potential wellapproaches saturation. The grids may be pulsed at a constant frequency,such as times per second, or may be pulsed for only a few times. Amultiple number of pulses, rather than one pulse, is preferably producedwhen the ion source is used in conjunction with a mass spectrometer tofacilitate visual inspection of the pulses on an oscilloscope and toaverage out statistical fluctuations in pulse amplitudes if present.

Although this invention has been disclosed and illustrated withreference to particular applications, the principles involved aresusceptible of numerous other applications which will be apparent topersons skilled in the art. The invention is, therefore, to be limitedonly as indicated by the scope of the appended claims.

What is claimed is:

1. An ion source, including, means for forming an electron stream, meansfor introducing a plurality of molecules into the electron stream forionization by the electrons and for retention in the potential wellcreated by the electron stream, means for cutting off the electronstream, and means for imposing a pulsed force upon the ions at apredetermined time relative to the cutting oif of the electron stream toremove the ions in a pulse from, the potential well.

2. An ion source, including, means for forming an electron stream, meansfor introducing a plurality of molecules into the electron stream, forionization by the electrons and for retention in the electric fieldcreated by the electron stream, means for periodically cutting ofi theelectron stream, and means for applying a pulsed field upon the ions ata predetermined time relative to each cutting off of the electron streamto remove the ions in a pulse from the area defined by the electronstream.

3. An ion source, including, means for producing a plurality ofelectrons, means for channeling the electron flow, means for introducinga plurality of molecules into the electron channel for ionization by theelectrons and for retention in the electron channel, means for cuttingoif the flow of electrons when the number of ions in the electronchannel approaches saturation, and means for applying an electric fieldof brief duration upon the ions at a predetermined time relative to thecutting off of the electron flow to remove the ions in'a bunch from theelectron channel.

4. An ion source, including, means for producing an electron stream,means for introducing molecules of the different gases in an unknownmixture into the stream for ionization by the electrons in the streamand for retention in the potential well created by the electron stream,means for considerably reducing the strength of the electron stream, andmeans for subjecting the ions to a pulsed electric field to withdraw theions in a bunch from the potential well at a predetermined time relativeto the reduction in the strength of the electron stream.

5. An ion source, including, means for producing a plurality ofelectrons, means for channeling the electron flow, means for introducinginto the electron channel molecules of the different gases in an unknownmixture for ionization by the electrons'in the channel and for retentionin the electron channel, means for cutting 01f the flow of electrons atpredetermined times, and means for subject ing the ions to a pulsedfield at approximately each cutoff of the electron flow for withdrawingthe ions in a bunch from the channel.

-6. An ion source, including, a filament adapted to emit a plurality ofelectrons, a control grid spaced from the filament and having a voltageto attract the electrons from the filament, there being a hole in thecontrol grid to pass the electrons in a directed stream, a collectorplate for receiving the stream of electrons, means for directing intothe electron stream a plurality of molecules of the dilferent moleculesin an unknown mixture forionization by, and retention in, theelectronstream, means for applying a voltage pulse between the control grid andthe filament to cut on the electron stream, an ion accelerator gridtransversely disposed relative to the control grid, and

meansfor applying a voltage pulse on the ion accelerator grid at apredetermined time relative to the pulse on the control grid to withdrawthe ions in a bunch.

7. An ion source, including, a filament adapted to emit a plurality ofelectrons, a control grid spaced from the filament, anelectronaccelerator grid spaced from the control grid, there being holesin the control and accelerator grids in alignment with the filament tocreate a beam of electrons, a collector plate spaced from the electronaccelerator grid in alignment with" the grid, means ior applyingvoltages on the filament, the control andaccelerator grids and thecollector plate to produce an accelerated travel of the electronstowards the collector plate, means for introducing a plurality ofmolecules into the potential well created by the electron beam forionization by the beam and for retention within. the space occupied bythe beam, an ion accelerator grid disposed in transverse relationship tothe electron accelerator grid, means for applying a voltage pulsebetween the control grid and the filament to cut off the how ofelectrons to the collector plate, and means for applying a voltage pulseto the ion accelerator grid at a predetermined time relative to theapplication of the voltage pulse on the control grid to attract the ionsin a bunch from the potential well towards the ion accelerator grid.

8. An ion source, including, means for forming a plurality of electrons,a collector, means for directing the electrons in a stream towards thecollector, means for producing a plurality of ions and for storing theions in the electron stream, means for reducing the strength of theelectron stream, and means for subjecting the ions to a pulsed field towithdraw the ions in a pulse in a direction transverse to that of theelectron stream.

9.- An ion source, including, a filament for emitting a plurality ofelectrons, a collector for receiving the electrons emitted from thefilament, a backing plate, a grid disposed relative to the backing plateto hold the electron stream between it and the plate, means forintroducing a plurality of molecules into the region between the plateand the grid for ionization by the electron stream and for retention ofthe ions within the stream, means for reducing' the strength of theelectron stream, and means for applying a pulsed field between thebacking plate and the grid upon the reduction in the strength of theelectron stream to impose an accelerating force upon the ions for thewithdrawal of the ions in a pulse from their place of retention.

10. An ion source, including, a filament for emitting a plurality ofelectrons, a collector for receiving the electrons emitted from thefilament, a backing plate, a grid disposed in substantial alignment withthe backing plate and on the opposite side of the electron streamrelative to the backing plate, means for introducing a plurality ofmolecules into the region between the plate and the grid for ionizationby the electron stream and for retention of the ions within the stream,means for reducing the strength of the electron stream, and means forapplying a voltage pulse to the grid upon the reduction in the strengthof the electron stream to impose an electric field between the plate andthe grid for the withdrawal of the ions in a pulse from their place ofretention.

11. An ionsource, including, means for forming a plurality of electrons,means for channeling the electron flow, means for introducing into theelectron channel molecules of the different gases in an unknown mixturefor ionization by the electrons in the channel and for retention in thechannel, means for reducing the strength of the electron channel, andmeans operative upon the reduction in the strength of the electronchannel to impose a pulsed electric field uponthe ions for thewithdrawal of the ions in a pulsev from their place of retention.

12'. An ion source, including, means for forming, a plurality ofelectrons, means for channeling the electrons intoa stream, means forintroducing a plurality of molecules into. the electron stream forionizationiby, andret'ent-ion within, the stream,. means for reducingthe strength of the stream, and means for imposing an accelerating forceon the ions for a short period of time, upon the reduction in thestrength of the stream, to Withdraw the ions in a pulse from their placeof retention.

13. Anion source, including, a filament for emitting a plurality ofelectrons, a collector for receiving the electrons emitted from thefilament, a first grid for accelerating the electrons towards thecollector and for confining them in anarrow stream, a backing plate, asecond grid disposed on the far side of the electron stream from thebacking plate, means. for introducing a plurality of. molecules into theelectron stream for ionization by the electrons and for retention of theions within the stream,

means for reducing the strength of the electron stream,

and means operative upon the reduction in the strength of the electronstream to impose pulses of voltage on the backing plate and the secondgrid to withdraw the ions in a pulse from their place of retention.

14. An ion source, including, means for producing a plurality ofelectrons, means for channeling the electron flow, means for introducinginto the electron channel molecules of the different gases in an unknownmixture for ionization by the electrons in the channel and for retentionof the ions in the channel, means for considerably reducing the flow ofelectrons periodically, and means for subjecting the ions to a pulsedelectric field at approximately each reduction in the flow of electronsto withdraw the ions in a bunch from their place of storagev and in adirection transverse to that of the electron channel.

15. Anion source, including, a filament for emitting a plurality ofelectrons, a collector for receiving the electrons emitted from thefilament, a first grid for channeling the flow of electrons into a beam,means for introducing a plurality of gas molecules into the electronbeam for ionization by the beam and for retention in the beam, a backingplate disposed in substantially parallel relationship to the beam, asecond grid disposed in substantially parallel relationship to thebacking plate on the other side of the beam with respect'to' the backingplate, means for cutting off the electron beam, and means for applyingvoltage pulses to the second grid upon the cut-off of the electron beamto withdraw thev ions in. a pulse from their place of retention and in adirection substantially perpendicular to the beam.

16. An ion source, including, means for forming a plurality ofelectrons, means for channeling the electron flow, means for introducinga plurality of molecules into the channel for ionization by theelectrons in the channel and for retention in the channel, andelectrical circuits for reducing the strength of the electron channelandfor imposing a pulsed electric field upon the ions for the withdrawalof the ions inv a pulse from their place of retention.

17. An ion source, including, a filament for emitting a plurality ofelectrons, a first grid for accelerating the electrons and for confiningthem in a narrow stream, a backing plate disposed on one side of theelectron stream, a second grid disposed on the other side of theelectron stream, means for introducing a plurality of molecules intotheelectron stream for ionization by the electrons and for retention of theions within the stream, and electrical circuits for reducing thestrength of the electron stream and for imposing pulses of voltage onthe backing plate relative to the voltage on the second grid to withdrawthe ions in a pulse from their place of retention.

18. An ion source, including, a filament for emitting a plurality ofelectrons, a collector for receiving the electrons, a first grid foraccelerating the electrons towards the collector and for confining themin a narrow stream,

.a backing plate disposed on one side ot the electron:

stream, a second grid disposed on the other side of the electron stream,means for introducing a plurality of molecules into the electron streamfor ionization by the electrons and for retention of the ions within thestream, and electrical circuits for reducing the strength of theelectron stream and for producing a pulsed electric field between thebacking plate and the second grid to with draw the ions in a pulse fromtheir place of retention.

19. An ion source, including, a filament for emitting a plurality ofelectrons, a first grid for channeling the flow of electrons into abeam, means for introducing a plurality of molecules into the electronbeam for ionization by the beam and for retention of the ions in thebeam, a backing plate disposed on one side of the electron beam and insubstantially aligned relationship to the beam, a second grid disposedon the other side of the electron beam and in substantially alignedrelationship with the beam, and electrical circuits for cutting off theelectron beam and for imposing a pulse of voltage on the second gridrelative to the backing plate to withdraw the ions in a pulse from theirplace of retention after the interruption of the beam and in a directionsubstantially perpendicular 20 to the beam.

20. An ion source, including, means for forming an electron stream in asubstantially field free region, means for introducing a plurality ofmolecules into the electron stream for ionization by the electrons andfor retention of the ions in the potential well created by the electronstream, means for cutting ofi the electron stream upon the retention ofa particular amount of ions in the potential well created by theelectron stream, and means for subjecting the ions to a pulsed field ata particular time relative to the cutting olf of the electron stream toproduce a movement of the ions in a pulse from their place of retention.

21. An ion source, including, means for forming an electron stream in asubstantially field free region, means for introducing a plurality ofmolecules into the electron stream for ionization by the electrons andfor retention of the ions in the potential Well created in the fieldfree region by the electron stream, means for considerably reducing thestrength of the electron stream, and means for applying to the ions apulsed electric field at a particular time after the reduction in thestrength of the electron stream to produce a movement of the ions in apulse from their place of retention.

References Cited in the file of this patent UNITED STATES PATENTS2,400,557 Lawlor May 21, 1946 2,457,162 Langmuir Dec. 28, 1948 2,570,124Hernqvist Oct. 2, 1951 2,582,216 Koppius Jan. 15, 1952 2,612,607Stephens Sept. 30, 1952

